Thermoset acrylic resins are widely used in producing ing coatings which have good flexibility and impact resistance. High solids coatings are of particular value to the automotive industry. Requirements for such coatings include toughness, flexibility, hardness and impact resistance.
Additionally, as environmental pollution standards controlling emissions of volatile organic compounds (VOC) have become increasingly more stringent over the past several years, coating producers have moved into the area of high solids thermoset coatings to attempt to meet such requirements. The limitation to higher solids in many cases is the spray viscosity which is the maximum viscosity the equipment utilized can tolerate. Thus, the need for even higher solids without viscosity increase is a very immediate and essential need of the industry. Further, with the advent and increasing use of plastic parts, more flexible coatings need to be designed without sacrificing the desired properties. Also, foreign competition in the automotive industry in particular has forced the U.S. industry to offer 5 and 10 year guarantees on automotive coatings. This has made industry require tougher coatings. In addition better chip resistant coatings, which require more flexible type resins, are needed.
The present invention is a method for improving flexibility and impact resistance of acrylic thermoset coatings and at the same time increasing solid levels that can be incorporated into a formulation at a specified reduced spray viscosity by adding a "co-reactive" monomer to a polymerization mix or, alternatively, by blending a "co-reactive" flexible polymer additive with a known rigid polymer to make a thermoset resin. The latter method of blending a "co-reactive" polymer is preferred because it provides a versatile and economic means of preparing acrylic thermoset coatings with varying degrees of toughness, flexibility, solid levels, hardness, and crosslink density to meet a wide variety of specific needs without redesigning a copolymer type resin for each need.
Compositions of the present invention are provided by reacting an alkylene oxide-hydroxyalkyl (meth)acrylate adduct, wherein the alkylene and hydroxyalkyl groups may each contain 2-8 carbon atoms, together with other polymerizable monomers. The adducts are made in the manner described in U.S. Pat. Nos. 3,312,654 and 4,126,527. The former '654 patent employs these adducts by reacting them with other polymerizable monomers, e.g. styrene or acrylonitrile; while the latter '527 patent employs such adducts to form polyacrylates by reaction with polyisocyanates or polycarboxylic acid polyanhydrides to provide plurality of ethylenically unsaturated acrylic acid ester groups which are used in radiation curable coating compositions.
Another patent, U.S. Pat. No. 3,773,710, teaches the use of physical blends of hard acrylic polymers with soft acrylic polymers. The former have a glass transition temperature &gt;-20.degree. C., while the latter have one &gt;-20.degree. C. Blends in which the glass transition temperatures of the hard and soft components differ by at least 30.degree. C. are said to give a good flexible, durable coating when mixed with a heat reactive condensate, e.g. a methylated melamine formaldehyde resin, to form a thermoset coating. The soft polymer which is added to the hard polymer in an amount within the range of 5-40% by weight is a copolymer of an alkyl (meth)acrylate with a hydroxyalkyl (meth)acrylate wherein the hydroxyalkyl ester amounts to 5-20% of the copolymer. The hard polymer may also contain 5-20% of hydroxyalkyl ester, the remainder being alkyl (meth)acrylates or styrene and mixtures thereof.